1. Field of the Invention
The present invention concerns a magnetic resonance system having a basic field magnet, a gradient system, a primary RF transmitting antenna, at least one secondary RF transmitting antenna, at least one RF receiving antenna and a control and evaluation device for operating the gradient system and the antennas and for evaluating he signals therefrom.
2. Description of the Prior Art
Magnetic resonance systems are generally known. The text “Bildgebende Systeme für die medizinische Diagnostik”, 3rd edition, 1995, Publicis MCD Verlag, pages 501 through 503, is referenced as an example of a general description of such systems.
The known resonance systems exhibit a basic magnetic field that is normally 1.5 T. A good reconstruction of the examination subject is possible with this magnetic resonance system.
In earlier times, magnetic resonance systems were also produced in which the basic magnetic field exhibited more than 1.5 T, in particular 3 T and more. Better resolutions are achievable with these magnetic resonance systems. However, image inhomogeneities that are caused by radio-frequency eddy currents in the examination subject increasingly occur with such magnetic resonance systems in the prior art.
In the prior art—, for example, U.S. Pat. No. 6,252,403—it is proposed to fashion the transmitting antenna spiral-shaped to compensate such eddy currents. The transmitting antenna thus assumes the shape of a birdcage resonator wound around its axis of symmetry. It is also known from this patent to arrange suitable dielectrics, in particular water, near the examination subject in order to hereby achieve a homogenization of the radio-frequency excitation field. In spite of such endeavors, however, inhomogeneities in the excitation field cannot be sufficiently compensated in all cases.
A magnetic resonance system according to the preamble of the claim 1 is known from U.S. Pat. No. 4,682,112 that has at least one control element that allows setting or adjustment of the amplitude ratio and/or phase offset of the respective excitation currents in the primary and secondary RF antennas. In this magnetic resonance system, the transmitter arrangement is comprised of an array of identically fashioned transmitter antennas that are separated axially and/or in the circumferential direction from one another. They can be individually activated by the control and evaluation device.
A magnetic resonance system having a number of receiving antennas that can be individually activated is known from U.S. Pat. No. 5,216,367.
A magnetic resonance system in which the transmitting antenna is fashioned as a modified birdcage resonator is known from U.S. Pat. No. 5,642,048.
From the scientific paper “Reduction of RF Penetration Effects in High Field Imaging” by Thomas K. F. Foo, Cecil E. Hayes and Yoon-Won Kang, appearing in Magnetic Resonance in Medicine 23 (1992), issue 2, pages 287 through 301, it is known to change the dielectric coupling of the radio-frequency shield of a whole-body transmitting antenna in order to achieve an optimally homogenous radio-frequency field within an excitation volume.